Coiled tubing is widely used in the oil and gas industry for a variety of purposes and applications, including, but not limited to, drilling, completion, and workover operations. For example, coiled tubing may be run into a subterranean well to produce hydrocarbons from the subterranean formation, to fracture or perforate the subterranean formation, to perform well data acquisition, introduce fluids, and to cleanout the wellbore.
Coiled tubing is typically supplied to the oilfield on a large spool or reel that contains is thousands of feet of continuous, relatively thin-walled tubing that typically has an outside diameter between about 1″ to 4.5″. During use, the tubing is payed or spooled off the reel and onto a device or “gooseneck” that bends and guides the coiled tubing into another device, such as an injector head. The injector head functions to grip the tubing and mechanically force it into, and withdraw it from, the wellbore.
Conventionally, coiled tubing injector heads employ motor driven endless chain loops that are supplied with gripper blocks for creating a strong friction grip against the coiled tubing. As the tubing is fed into the injector head, the gripper blocks press against the coiled tubing, which is mechanically forced thereby into the wellbore as the endless chains of the injector head are turned. The direction of the endless chain loops may be reversed to withdraw the tubing from the wellbore. The coiled tubing is conventionally introduced into the wellbore through a seal, which contains the well pressure as the coiled tubing is introduced or withdrawn.
The coiled tubing injector head is conventionally positioned above the wellhead. In workover operations, for example, the injector head maybe suspended above the wellbore by a crane or other device. A tubing guide may be used to connect the injector head to the wellhead (including, for example, a blowout preventer) at the top of the wellbore to prevent the coiled tubing from buckling or otherwise deforming prior to entering the wellbore.
It is well known that unspooling and re-spooling coiled tubing and running the coiled tubing into and out of a wellbore, causes the coiled tubing to experience a number of bends that may contribute to fatigue crack initiation. For example, in one trip in and out of the well, the coiled tubing may experience at least six different bending events: bending off the reel, onto the gooseneck, off the gooseneck into the injector, then from the injector onto the gooseneck, off the gooseneck toward the reel, and finally rolling back onto the reel itself. The number of fatigue cycles that a bending event causes depends on various factors including the coiled tubing material, the coiled tubing geometry, the pressure, the bending radius, and the current state of fatigue already existing in the coiled tubing.
At some point, repeated bending may initiate a micro fracture that left unchecked can propagate to catastrophic fatigue failure of the coiled tubing. Because failure by fatigue cannot is be predicted with perfect accuracy, operators typically use a reel of coiled tubing to no more than approximately 80% of its estimated useful fatigue life. Thus, the industry tries to count the bending events to which a reel of coiled tubing is subjected.
There are several industry approaches to “counting” the number of bends and unbends, or fatigue cycles, that a spool of coiled tubing experiences during its life. For example, in October 1996, Maurer Engineering, Inc. (Houston, Tex.) published a paper entitled “Coiled-Tubing Fatigue Model (CTLIFE2)/Theory and User's Manual/DEA-67, Phase II,” which was distributed at the participants of the Drilling Engineering Association DEA-67, Project to Develop and Evaluate Coiled-Tubing and Slim-Hole Technology, which paper is incorporated herein for all purposes. (A copy of this published paper can presently be found at http://www.bsee.gov/Research-and-Training/Technology-Assessment-and-Research/tarprojects/300-399/300AQ.aspx). In May 19, 1999, Professor Steven M. Tipton of The University of Tulsa published a Tech Note (entitled “The Achilles Fatigue Model”) on his Achilles 3.0 fatigue model for the Cerberus coiled tubing modeling software, which Tech Note is incorporated herein for all purposes. (A copy of this Tech Note can presently be found at http://www.google.com/url?sa=t&rct=j&q=&esrc=s&frm=1&source=web&cd=1&cad=rja&ved=0CC0QFjAA&url=http%3A%2F%2Fctes.nov.com%2Fdocumentation%2Ftechnotes%2FTech%2520Note%2520Achilles%2520Fatigue%2520Model.pdf&ei=nQzSUab0JoXS9ASSqYGoDA&usg=AFQjCNFSJXfzaZQ5_JmmmsqPB2Xb_SQAHg&sig2=O2gQrQ-E0jnSTGrmupyIzA.) Thus, conventional wisdom teaches that a spool of coiled tubing has a finite life that is a function of the number of bend cycles that the coiled tubing is subjected to.
It is desirable, therefore, to reduce the number of bending cycles that a spool of coiled tubing is subjected to during normal use. One approach that has been used to minimize tubing fatigue arising from short movements of the coiled tubing is set forth in U.S. Pat. No. 6,457,534. However, the approach of U.S. Pat. No. 6,457,534 requires raising the coiled tubing off of the gooseneck, which can lead to instability of the coiled tubing string and difficulty in realigning the coiled tubing string back onto the gooseneck.
Accordingly, there is a need for an improved method and apparatus for axially displacing coiled tubing while minimizing fatigue.